In the operation of an electric power transmission and distribution system, it is possible that abnormal and possibly dangerous conditions may arise which are not readily detectable by any practical monitoring system available today. One such condition is that of an energized high voltage conductor lying on a pavement or upon dry earth. This could result from the breakage of a utility pole, which normally carries the energized wire beyond reach of a person. Such condition could arise by the pole being broken or downed by a vehicle or by severe weather. Sometimes, a conductor splice connecting two energized conductors together might fail, allowing one or both of the conductor ends to fall upon dry ground or upon pavement.
Frequently, in such cases, the amount of fault current passing from the energized wire to ground is quite limited, due to the high impedance in the ground return path. Conventional means for detecting down wires and the like include known types of short circuit detection equipment, such as current monitoring, circuit breakers, fuses and the like. Such conventional means are often not sufficiently sensitive to detect the small fault current resulting from a high impedance fault.
In such cases, the high impedance fault current may be only a few percent of the normal load current, and thus is not likely to be recognized by protective devices commonly used in electric power distribution systems, such as protective relays, circuit breakers or fuses. A most difficult down conductor condition to recognize is that in which the conductor breaks and lies on a dry asphalt pavement, and there is substantially no fault current at all.
It is known that a high impedance fault in an electric power distribution system is often the result of either an unstable arc, or of an arc of substantial length, or is the result of a high resistance, dry earth ground return fault path. In any of these cases, the high impedance which limits the fault current will be generally nonlinear during the voltage excursion of the distribution system during each half cycle. It is known that this results in a distorted system current wave which, upon analysis, can be shown to contain harmonic frequencies as well as the fundamental frequency component of the electric power distribution system.
It has been proposed to simply measure these harmonic currents on the electric power distribution system. Such an approach, however, is not generally sufficient to identify positively the existence of such a fault condition. One reason for this is that, on a typical commercial electric power transmission and distribution network, the sources of harmonics are continually changing, and thus the harmonics normally present on the system are changing as well.
Among the sources of harmonics normally occurring in the system are the many transformers and electric motors connected to the system, ballast type lamps, arc furnaces, rectifiers, thyristor controlled loads, television sets and other energized nonlinear devices. Harmonic sinks include all the components connected to the system, including even some of those components which are themselves harmonic sources.
Variations in both the sources and the sinks, or absorbers, of normally occurring harmonics on the power distribution system, can mask harmonics which may arise in response to the occurrence of a high impedance fault or other abnormality.
A characteristic common to many of these harmonic sources is that they are nonlinear inductances, or that they, by their nature, require an inductive current, such as in the case of the electric arc furnaces mentioned above.
The exciting current of a typical transformer, illustrated in the waveform of Figure 1A illustrates that some of these distorted currents tend to peak when the voltage is near its zero value. A distorted, flat-top current wave similar to the square wave illustrated in FIG. 1b is drawn by a single phase thyristor controlled motor load.
It is an object of this invention to provide a system for analyzing conditions on an electric power system to detect the occurrence of high impedance faults on the system, without producing spurious indications in response to normal, non-fault changes in the system characteristics which result from normal changes in system load and conditions.